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Versions: 00                                                            
HTTPbis                                                          M. West
Internet-Draft                                               Google, Inc
Updates: 6265 (if approved)                                 June 6, 2017
Intended status: Standards Track
Expires: December 8, 2017

                            Non-HTTP Cookies


   This document updates RFC6265 by defining a "NonHttp" attribute which
   ensures that a given cookie is available only to "non-HTTP" APIs.
   Yes, it is a little strange for "HTTP State Management" to support
   exclusively non-HTTP use cases, but the internet is a strange place.
   So it goes.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 8, 2017.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Examples  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology and notation  . . . . . . . . . . . . . . . . . .   3
   3.  Recommendations . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Security and Privacy Considerations . . . . . . . . . . . . .   5
     4.1.  Cross-origin storage capabilities . . . . . . . . . . . .   5
     4.2.  Fewer Cookies on the Wire . . . . . . . . . . . . . . . .   5
     4.3.  Legacy Clients  . . . . . . . . . . . . . . . . . . . . .   6
     4.4.  Other Constraints . . . . . . . . . . . . . . . . . . . .   6
   5.  Aesthetic Considerations  . . . . . . . . . . . . . . . . . .   6
     5.1.  Isn't 'DOMOnly' a better name? Or 'NoHttp'? Or 'Insert
           Bikeshed Here'? . . . . . . . . . . . . . . . . . . . . .   6
   6.  Normative References  . . . . . . . . . . . . . . . . . . . .   6
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   Cookies violate the same-origin policy in well-understood ways,
   outlined in Sections 8.5 and 8.6 of [RFC6265].  For better or worse,
   developers rely on this behavior.  In particular, it is quite common
   for developers to use cookies as a cross-origin storage mechanism,
   allowing state to be shared across all origins in a given registrable
   domain.  A user who signs into "https://m.example.com/" might expect
   to also be signed into "https://www.example.com/", and vice-versa.
   Servers can support this expectation by setting a cookie at the apex
   domain: "uid=1234567;Secure;Domain=example.com".  The cookie will be
   sent along with HTTP requests to both origins, and the user's state
   can be maintained server-side.

   Often, though, the fact that a cookie is sent over the wire is an
   unindended side-effect.  Developers may be using cookies to easily
   share client-side state between related origins, and have pressed
   cookies into this service due to their unique security properties.

   For example, web analytics packages might use a user agent's
   "document.cookie" API to set first-party cookies on a client's site
   that retain a user's state.  This gives them the information they
   need to do analytics work, but has the very unfortunate side-effect
   of causing those cookies to be sent along with requests to the first-
   party's server.  This has a substantially negative impact on request
   size (and therefore performance), and also on privacy, as these

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   cookies often contain identifiers, and are often sent in plaintext
   over the network.

   This document aims to reduce both impacts by allowing developers to
   specify specific cookies as "NonHttp".  These cookies cannot be set
   or modified via "Set-Cookie" HTTP response headers, nor will they be
   included in "Cookie" HTTP request headers.  They are only accessible
   via "non-HTTP" APIs, satisfying purely client-side storage

1.1.  Examples

   Non-HTTP cookies are only accessible via "non-HTTP" APIs.  For
   example, a developer might use HTML's "document.cookie" to set a non-
   HTTP cookie as follows:

   document.cookie = "name=value; Secure; NonHttp";

   That cookie would be available via subsequent calls to
   "document.cookie", but will not be included as a "Cookie" header in
   HTTP requests to the developer's server.

   The same cookie would not be accepted if set via a "Set-Cookie"
   header.  That is, the following header would result in no change to
   the user agent's cookie store:

   Set-Cookie: name=value; Secure; NonHttp

   Likewise, cookies that contain both the "NonHttp" and "HttpOnly"
   flags will be ignored.  The following is a no-op:

   document.cookie = "name=value; Secure; NonHttp; HttpOnly"

   As is the following:

   Set-Cookie: name=value; Secure; NonHttp; HttpOnly

2.  Terminology and notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

3.  Recommendations

   This section describes extensions to [RFC6265] necessary to implement
   the "NonHttp" attribute.

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   1.  Extend the "cookie-av" grammar in Section 4.1.1 of [RFC6265] as

   cookie-av      = expires-av / max-age-av / domain-av /
                    path-av / secure-av / httponly-av /
                    nohttp-av / extension-av
   nohttp-av      = "NoHttp"

   2.  Add a section to Section 4.1.2 of [RFC6265] describing the
       semantics of the "NonHttp" attribute as follows:

       The "NonHttp" attribute limits the scope of the cookie to "non-
       HTTP" APIs.  That is, the attribute instructs the user agent to
       omit the cookie when constructing a "Cookie" header for an HTTP

       If both the "NonHttp" and "HttpOnly" attributes are present when
       setting a cookie, the cookie will be ignored, regardless of its
       delivery mechanism.

   3.  Alter the storage model defined in Section 5.3 of [RFC6265] as

       1.  Add "no-http-flag" as a field to be stored on each cookie.

       2.  Insert the following two steps after the current step 10:

           1.  If the "cookie-attribute-list" contains an attribute with
               an "attribute-name" of "NoHttp", set the cookie's "no-
               http-flag" to "true".  Otherwise, set the cookie's "no-
               http-flag" to "false".

           2.  If the cookie was not received from a "non-HTTP" API, and
               the cookie's "no-http-flag" is "true", abort these steps
               and ignore the cookie entirely.

       3.  Insert the following step after the current step 11.2:

           1.  If the newly created cookie was not received from a "non-
               HTTP" API, and the "old-cookie"'s "no-http-flag" is
               "true", abort these steps and ignore the newly created
               cookie entirely.

   4.  Add a section to Section 5.2 of [RFC6265] describing the
       processing requirements for the "NonHttp" attribute as follows:

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       If the "attribute-name" case-insensitively matches the string
       "NonHttp", the user agent MUST append an attribute to the
       "cookie-attribute-list" with an "attribute-name" of "NonHttp" and
       an empty "attribute-value".

   5.  Alter the "Cookie" header generation in Section 5.4 of [RFC6265]
       as follows:

       1.  Add the following requirement to the list of requirements in
           the current step 1:

           +  If the cookie's "no-http-flag" is "true", then exclude the
              cookie if the "cookie-string" is not being generated for a
              "non-HTTP" API (as defined by the user agent).

4.  Security and Privacy Considerations

4.1.  Cross-origin storage capabilities

   There's a risk that allowing developers to suppress cookies from HTTP
   requests might lead to increased usage of cookies as a cross-origin
   storage mechanism.  Given existing usage, however, the worst case
   seems to be an increase from a high number to a marginally higher

   Note, though, that the capabilities provided here can already be
   obtained through clever use of the "path" attribute to scope cookies
   down to paths that aren't frequently requested.  These cookies can be
   accessed through clever use of "<iframe>" elements and the
   "history.pushState" API.

   The flag introduced here makes that mechanism significantly simpler,
   but does not introduce fundamentally new capabilities.

4.2.  Fewer Cookies on the Wire

   Non-HTTP cookies are never sent directly by the user agent over the
   network, which reduces their exposure to both active and passive
   network attackers.  It seems reasonable to expect that adoption of
   the "NonHttp" attribute by popular analytics packages could result in
   a substantial reduction in the usefulness of those packages' cookies
   when attempting to correlate a given user's activities over time and
   across networks.

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4.3.  Legacy Clients

   Clients that do not support the "NonHttp" flag will fall back to the
   existing behavior: cookies with the flag will be accepted via any
   delivery mechanism, and will continue to be sent out with matching
   HTTP requests.

   Developers can use this behavior to feature-detect support for the
   flag: sending "Set-Cookie: nonhttp-support=0; NonHttp" will allow
   server-side detection by parsing the "Cookie" header sent with
   requests, and client-side detection via code like

4.4.  Other Constraints

   Whether or not a cookie is flagged as "NonHttp" MUST not affect
   eviction behavior in general.  Cookies are cookies, no matter their
   flags, so the "remove excess cookies" constraints, expiration
   behavior, and "persistent-flag" behavior discussed in the current
   step 12 of Section 5.3 of [RFC6265] would still apply.

5.  Aesthetic Considerations

5.1.  Isn't 'DOMOnly' a better name?  Or 'NoHttp'?  Or 'Insert Bikeshed

   "DOMOnly" is probably a better fit for the practical implications of
   this API.  "DocumentCookieOnly" is even more clear.  This document
   runs with "NonHttp" to match RFC 6265's existing language around
   "non-HTTP" APIs, and to avoid arguments about software that handles
   cookies, but doesn't work with a DOM or any particular JavaScript
   interface.  Bikeshedding is, of course, welcome.

6.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC6265]  Barth, A., "HTTP State Management Mechanism", RFC 6265,
              DOI 10.17487/RFC6265, April 2011,

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Appendix A.  Acknowledgements

   Michael Nordman suggested this approach during a conversation about
   cross-origin storage mechanisms.  Brad Townsend helped me understand
   the potential positive impact on traffic levels for analytics

Author's Address

   Mike West
   Google, Inc

   Email: mkwst@google.com
   URI:   https://mikewest.org/

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